The present invention relates to a rotary hydraulic device. In particular, the present invention relates to a hydraulic pump or motor including a rotor carrying a number of piston elements around its periphery, and a cam ring enclosing the rotor for causing the piston elements to move along a stroke length as the rotor rotates.
Many industrial and automotive devices require a continuous supply of fluid, such as oil, fuel or hydraulic fluid, for proper operation. However, it is also desirable to be able to maintain or vary the delivery rate of the fluid as the application demands. To meet this need, two approaches have been taken:
1. a constant-capacity pump is driven by a prime mover, and the flow rate of the pump is varied by returning a portion of fluid from the output port of the pump back to the input port
2. a variable-capacity pump, including a fluid delivery piston, is driven by a prime mover, and the flow rate of the pump is altered by altering the stroke of the piston
The former approach makes inefficient use of the energy used to drive the pump since a portion of pressurized fluid is returned to the reservoir instead of performing useful work. On the other hand, the latter approach has been favoured because (1) variable-capacity pumps make more efficient use of energy, (2) the speed of the prime mover can vary without impacting on the flow rate of the variable-capacity pump, and (3) variable capacity pumps can alter their output flow rate more rapidly, in response to changes in operating conditions, than constant-capacity pumps.
The conventional variable-flow rotary pump comprises a hollow casing; a cam ring provided within the casing; and a rotor provided within the cam ring and being rotatably mounted about a fixed axis. The rotor includes a series of radial angularly-spaced fluid chambers disposed about its circumference, and a roller provided within each slot. The casing includes a fluid inlet port for delivering fluid to the fluid chambers, and a fluid outlet port for receiving pressurized fluid from the fluid chambers. Generally, the centre axis of the cam ring is displaced a distance from the fixed axis of the rotor. Consequently, as the rotor rotates, the volume of each fluid chamber will vary between minimum and maximum values as the respective roller moves between its innermost position and its outermost position. Further, the cam ring includes means for varying the position of the cam ring relative to the rotor. In one position, the centre of the cam ring is displaced a maximum distance from the fixed axis of the rotor, causing the communication time a fluid chamber of increasing volume is in communication with the inlet port to be maximized. In another position, the centre of the cam ring is displaced a minimum distance from the fixed axis of the rotor, causing the communication time a fluid chamber of increasing volume is in communication with the inlet port to be minimized. Consequently, the output flow rate of the pump can be varied between a maximum and a minimum value without varying the rotational speed of the rotor.
Numerous variations on the conventional variable-flow rotary pump have been developed. For instance, Wilcox (U.S. Pat. No. 3,381,622) teaches a variable-flow rotary pump having a constant output pressure. As shown in FIG. 1 of the patent, the pump comprises a mounting plate 20; a cavity body 30 mounted to the mounting plate 20; a cavity ring 31 provided within the cavity body 30; and a rotor 32 rotatably mounted about a fixed axis within the cavity ring 31. The rotor 32 includes a series of radial angularly-spaced slots 33, each including a pump roller 34. The mounting plate 20 includes an arcuate fluid inlet port 62 and an arcuate fluid outlet port 63 aligned with the root circle of the roller slots 33 for respectively delivering fluid to and removing fluid from each slot 33 as the rotor 32 rotates. The pump also includes a leaf spring 110, and a pressure conduit 91 coupled between the cavity ring 31 and the leaf spring 110 for reducing the eccentricity of the cavity ring (and hence the output pressure) as output pressure increases.
Bristow (U.S. Pat. No. 4,679,995) teaches avariable-flow rotary pump which is substantially similar to the variable-flow rotary pump taught by Wilcox, except that the cam ring 10 (equivalent to the cavity ring 31) is rotatably coupled at one end and to a transversely-extending spring 23 at the opposite end for urging the cam ring 10 into a maximum pump flow position. At the same time, a portion of the pressurized output fluid exerts a force opposite to the force exerted by the spring 23 so as to reduce the output flow of the pump when the output pressure increases.
Maistreli (U.S. Pat. No. 3,642,388) teaches a variable-capacity vane pump whose output flow is continuously variable. As shown in FIG. 2 of the patent, the vane pump comprises a hollow casing 1 including an inlet port 24 and an outlet port 25; a cam ring 9 provided within the casing 1; and a rotor 2 rotatably mounted about a fixed axis within the cam ring 9. The rotor 2 includes a series of radial angularly-spaced notches 6 each including a cylindrical roller. The cam ring 9 is rotatably coupled to a roller 41 at one end, and to a hydraulically-operated piston 11 at the opposite end for urging the ring 9 between a maximum pump flow position and a minimum pump flow position in response to changes in hydraulic fluid pressure delivered to the piston 11.
Hutson (U.S. Pat. No. 4,578,948) teaches a reversible-flow vane pump. As shown in FIGS. 3,4 and 5 of the patent, the pump comprises a pump case (not shown) including a first 76 and a second 78; an annular cam ring 40 provided within the pump case and being pivotable about a pin 44; and a rotor 20 rotatably mounted about a fixed axis within the cam ring 40. The rotor 20 includes a series of equally-spaced circumferential outwardly-opening slots 32, each including a roller vane 34 which engages the inner cam surface of the annular cam ring 40.
In the operating mode shown in FIG. 4 of the patent, the cam ring 40 is pivoted about pin 44 so as to increase the communication time a fluid chamber of increasing volume is in communication with the first port 76 and thereby cause a forward pump flow between ports 76 and 78, whereas in the operating mode shown in FIG. 5, the cam ring 40 is pivoted in an opposite direction about pin 44 so as to increase the communication time a fluid chamber of increasing volume is in communication with the s econd port 78 and thereby cause a reverse flow between ports 76 and 78 without reversing the direction of rotation of the rotor 20. In the operating mode shown in FIG. 3, the cam ring 40 is positioned so that the communication times of the fluid chambers in communication with the first port 76 is equal to the communication times of fluid chambers in communication with the second port 78. Consequently, in this latter position, there is no net fluid flow between the ports 76, 78.
Delegard (U.S. Pat. No. 2,612,110) describes a variable flow rotary pump which comprises an oval cam ring, a rotor disposed within the cam ring and having a number of pockets each retaining a piston therein, and end plates having fluid inlet and outlet ports in communication with the outermost portion of the pockets.
Grupen (U.S. Pat. No. 2,880,677) describes a variable volume vane pump which includes a stator provided with a symmetric oval through-bore, diametrically-opposed inlet ports and diametrically-opposed outlet ports opening into the bore via the radially outermost portion of the bore, and a set of uniformly-spaced slots opening into the periphery of the rotor each carrying a sliding vane which projects into the bore.
Each of the foregoing variations has addressed deficiencies of the conventional variable-flow rotary pump. However, in each variation, differences in the fluid pressures of the fluid chamber approaching the outlet port and the fluid chamber leaving the outlet port can cause unwanted ripples in the output pressure of the pump.
Attempts have also been made to control the output pressure of a rotary pump. Brighton (European Patent 0 841 485) describes a self-regulating rotary pump which includes an outer spacer ring, a flexible cam ring disposed within the spacer ring, a cavity disposed between the spacer ring and the cam ring, a rotor disposed within the cam ring and carrying a number of slots carrying pistons therein, and end plates having fluid outlet ports and fluid inlet ports aligned with the radially innermost portion of the slots. The cam ring includes a pair of apertures for bleeding pressurized fluid into the cavity regions and thereby deform the cam ring between a symmetric oval configuration and a circular configuration in response to variations in average output pressure.
Sipe (European Patent 0 200 294) discloses a rotary pump configured for reducing pressure pulsations in the discharge flow. The pump comprises a cam ring having an oval high displacement cam surface, and a rotor having a number of radially-extending slots each receiving a cylindrical vane member. Each slot includes a driving surface which has a surface portion which is configured to reduce the pressure pulsations by maintaining the roller vanes in contact with the cam surface.
Although both Brighton and Sipe attempt to reduce variations in output pressure, neither Brighton would not be able to reduce fluid cavitation, and the driving surface disclosed by Sipe would be difficult to manufacture. Accordingly, there remains a need for a rotary pump which provides a steady fluid output pressure and reduces the likelihood of fluid cavitation.
According to the present invention, there is provided a rotary pump which addresses the deficiency of the prior art.
The rotary pump, according to the present invention, comprises a cam ring, a rotor disposed within the cam ring, and a pump body enclosing the cam ring and the rotor. The cam ring includes a cam surface having a centre of symmetry. The rotor has a centre of rotation which coincides with the centre of symmetry of the cam surface, and includes a plurality of fluid chambers. Each fluid chamber comprises an aperture opening into a circumference of the rotor, and a pump element sealingly disposed within the aperture. As the rotor revolves, each element remains in contact with the cam surface and moves over a stroke length between a first position adjacent the radial innermost portion of the respective aperture and a second position adjacent the radial outermost portion of the respective aperture. The pump body includes a fluid inlet and a fluid outlet respectively for transferring fluid to and fluid from the fluid chambers as the rotor rotates. Preferably, the pump also includes an actuator for rotating the cam ring about its centre of symmetry between a first angular position and a second angular position for varying the stroke length of the pump elements.
In one embodiment of the invention, the cam surface comprises a number xe2x80x9cNxe2x80x9d (at least two) of cam lobes. The pump body includes an equal number of fluid inlets and fluid outlets, with the number of fluid inlets and the number of fluid outlets corresponding to the number xe2x80x9cNxe2x80x9d of cam lobes.
In another embodiment of the invention, the pump body includes an equal number xe2x80x9cNxe2x80x9d (at least two) of fluid inlets and fluid outlets, and the cam surface is shaped so that each pump element cycles over the stroke length xe2x80x9cNxe2x80x9d times per rotor revolution.